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Harvard University researchers have engineered a new fabrication method that promises to accelerate a lagging area of 3D printing technology – multimaterial printing.

The technique, called multimaterial multinozzle 3D printing (MM3D), was developed by researchers from Harvard’s Wyss Institute for Biologically Inspired Engineering and John A. Paulson School of Engineering and Applied Sciences (SEAS). MM3D uses high-speed pressure valves that continuously and seamlessly switch between up to eight different printing materials at a speed of up to 50 times per second. That's about as fast as a hummingbird can flap its wings, according to the researchers.

This enables the creation of complex shapes in a fraction of the time currently required using printheads that range from a single nozzle to large multinozzle arrays.

Moreover, the 3D printheads used in the process are also fabricated using 3D printing, which allows for customization and also could allow for others in the industry to create their own.

“When printing an object using a conventional extrusion-based 3D printer, the time required to print it scales cubically with the length of the object because the printing nozzle has to move in three dimensions rather than just one,” Mark Skylar-Scott, a research associate at the Wyss Institute said in a press statement.

MM3D’s combination of multinozzle array provides the ability to switch between multiple inks rapidly to eliminate the time lost to switching printheads, which helps to “get the scaling law down from cubic to linear, so you can print multimaterial, periodic 3D objects much more quickly,” Skylar-Scott said.

MM3D printing’s unique 3D-printed printhead design allows it to seamlessly switch between multiple different materials up to 50 times per second. (Image source: Wyss Institute at Harvard University

Overcoming droplet physics

Currently, most commercial 3D printers are only able to build objects from a single material at a time. While there are inkjet printers that are capable of multimaterial printing, these are constrained by the physics of droplet formation.

One method of 3D printing that does allow for the use of multiple materials to create one object is extrusion-based printing. However, this process is extremely slow. For example, it would take about 10 days to build a 3D object about one liter in volume at the resolution of a human hair and print speed of 10 cm/s using a single-nozzle, single-material printhead.

MM3D solves this speed issue with a series of Y-shaped junctions inside the printhead where multiple ink channels come together at a single output nozzle.

The Harvard team calculated and tuned the shape of the nozzle, printing pressure, and ink viscosity so that when pressure is applied to one of the “arms” of the junction, the ink that flows down through that arm doesn’t cause any of the other arms to spill ink. The researchers said this prevents the inks from mixing and preserves the quality of the printed objects.

The engineers also use a bank of fast pneumatic valves to operate the printheads, facilitating this one-way flow behavior so multimaterial filaments can be rapidly assembled and flow continuously out from each nozzle to construct a part.

The length of the ink channels can be adjusted to account for materials that have different viscosities and yield stresses, which would allow them to flow more quickly or slowly than other inks for custom multimaterial fabrication, said Jochen Mueller, a research fellow at Wyss and SEAS.

“Because MM3D printing can produce objects so quickly, one can use reactive materials whose properties change over time – such as epoxies, silicones, polyurethanes, or bio-inks,” he said in a press statement. “One can also readily integrate materials with disparate properties to create origami-like architectures or soft robots that contain both stiff and flexible elements.”

The researchers published a paper on their work in the journal Nature. In it they describe how they demonstrated their technique by printing a Miura origami structure composed of stiff “panel” sections connected by highly flexible “hinge” sections.

To build such an object using previously available methods would have required manually stacking layers of objects that were printed separately. By using MM3D the researchers could print the entire object in a single step by using eight nozzles to continuously extrude two alternating epoxy inks that achieved varying stiffnesses after being cured.

Moreover, researchers reported that the hinges withstood over 1,000 folding cycles before failing. This demonstrates that the transitions between stiff and flexible materials during printing was of a high quality, they said.

Another, more complex object researchers printed using MM3D was a soft robot composed of rigid and soft elastomers in a millipede-like pattern. The robot included embedded pneumatic channels that compressed its actuators sequentially using a vacuum, allowing the robot to “walk” while carrying a load eight times its weight.

The Harvard researchers aid they aim to continue to evolve the MM3D process in a number of ways, particularly to improve the nozzles used. They would like to develop nozzles that can extrude different inks at different times, smaller nozzles for greater resolution, and even larger arrays for rapid, single-step 3D printing that range in size and resolution scales. The researchers also are exploring how to achieve even more complex shapes using sacrificial inks.

Elizabeth Montalbano is a freelance writer who has written about technology and culture for more than 20 years. She has lived and worked as a professional journalist in Phoenix, San Francisco and New York City. In her free time she enjoys surfing, traveling, music, yoga and cooking. She currently resides in a village on the southwest coast of Portugal.

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